So-called prepackaged foods are rapidly widespread such that various foods are sealably packed in a packaging container such as a plastic bag and a plastic tray, and it is true to say that such prepackaged foods contribute to the food style of modem society to a great extent. It has been a common practice to heat the prepackaged foods for sterilization after a food is sealed in a packaging container. A certain heating process (heat maturation process) is also applied to a certain kind of food, combined with heat sterilization, to improve tastes and flavors of the food.
At a last stage of heat sterilization, boiling and steaming are generally performed. However, boiling and steaming require a long time for processing, which lowers sterilization efficiency, and in addition, the flavors of the food may be degraded. The applicants of this invention have proposed a high frequency heating apparatus in which plural prepackaged foods are loaded in a flat sealed container and a high frequency power is supplied to the container by way of opposing electrodes in a pressurized state (Japanese Patent Application No. HEI 7-329296).
In the above high frequency heating apparatus, inside the container where the prepackaged food is loaded is brought to a sealed state by the opposing electrodes in contact with a peripheral portion of the container. Accordingly, it is possible to heat the inside of the sealed container (food loading chamber) at a temperature of 100.degree. C. or higher to sterilize the food in a short time.
In the above high frequency heating apparatus, pressurizing means for pressurizing the inside of the container is necessary. Particularly, since plural prepackaged foods are loaded in the cylindrical shape container in the above apparatus, large size pressurizing means is necessary, which would raise the installation cost for the apparatus. Further, it is difficult to uniformly apply a high frequency to the plural prepackaged foods loaded in the container. Specifically, the heated temperature may vary depending on the loaded position of the prepackaged food which hinders uniform heating, and there may occur a situation that sterilization is not completed with respect to the prepackaged food that has been loaded at a lower temperature heating position. Incomplete sterilization eventually leads to non-sterilization due to growth of bacteria.
To enhance sterilizing ability, there is an idea of applying a high frequency to a prepackaged food for a longer time. However, this idea leads to energy cost rise which is not desirable in the economic viewpoint. Further, a new problem may be raised that the flavor of the prepackaged food is degraded due to over heating.
There have been proposed various sterilizing apparatuses by microwave radiation in which a prepackaged food is accommodated in a support housing member made of a synthetic resin having an inner configuration substantially equal to the outer shape of the prepackaged food (European Patent No. 0269073 (European Patent Application No. 8711732B.2), Japanese Examined Patent Publication No. HEI 7-51060, Japanese Examined Patent Publication No. HEI 7-114672, Japanese Utility Model Registrar No. 2539778, and Japanese Patent No. 2778165).
In view thereof, an object of this invention is to provide a prepackaged food sterilizing apparatus and a sterilizing method that enables assured sterilization to a prepackaged food while suppressing process cost for sterilization.